An organic aerosol particle has a lifetime of approximately 1 week in the
atmosphere during which it will be exposed to sunlight. However, the effect of
photochemistry on the propensity of organic matter to participate in the
initial cloud-forming steps is difficult to predict. In this study, we
quantify on a molecular scale the effect of photochemical exposure of
naturally occurring dissolved organic matter (DOM) and of a fulvic acid
standard on its cloud condensation nuclei (CCN) and ice nucleation (IN)
activity. We find that photochemical processing, equivalent to 4.6 d in
the atmosphere, of DOM increases its ability to form cloud droplets by up to
a factor of 2.5 but decreases its ability to form ice crystals at a loss
rate of -0.04∘CT50 h-1 of sunlight at ground level.
In other words, the ice nucleation activity of photooxidized DOM can require
up to 4 ∘C colder temperatures for 50 % of the droplets to activate
as ice crystals under immersion freezing conditions. This temperature change
could impact the ratio of ice to water droplets within a mixed-phase
cloud by delaying the onset of glaciation and by increasing the supercooled
liquid fraction of the cloud, thereby modifying the radiative properties and
the lifetime of the cloud. Concurrently, a photomineralization mechanism was
quantified by monitoring the loss of organic carbon and the simultaneous
production of organic acids, such as formic, acetic, oxalic and pyruvic
acids, CO and CO2. This mechanism explains and predicts the observed
increase in CCN and decrease in IN efficiencies. Indeed, we show that
photochemical processing can be a dominant atmospheric ageing process,
impacting CCN and IN efficiencies and concentrations. Photomineralization
can thus alter the aerosol–cloud radiative effects of organic matter by
modifying the supercooled-liquid-water-to-ice-crystal ratio in mixed-phase
clouds with implications for cloud lifetime, precipitation patterns and the
hydrological cycle.Highlights.
During atmospheric transport, dissolved organic matter (DOM) within aqueous
aerosols undergoes photochemistry. We find that photochemical processing of
DOM increases its ability to form cloud droplets but decreases its ability
to form ice crystals over a simulated 4.6 d in the atmosphere. A
photomineralization mechanism involving the loss of organic carbon and the
production of organic acids, CO and CO2 explains the observed changes
and affects the liquid-water-to-ice ratio in clouds.
Introduction
Aerosol–cloud interactions play a key role in the earth's energy budget yet
contribute to the largest uncertainty in radiative forcing in climate model
estimates (Boucher et al.,
2013). Mixed-phase clouds are particularly interesting because of their
predominant role in global precipitation and their unstable microphysics due
to the coexistence of liquid water and ice. The presence of liquid water in
mixed-phase clouds is due to the ability of an aerosol particle to activate
into a cloud droplet in the presence of high relative humidity, as predicted
by κ-Köhler theory (Farmer et al., 2015;
Petters and Kreidenweis, 2007). First the size and second the chemical
composition of aerosol particles are the key factors controlling their
ability to act as cloud condensation nuclei (CCN)
(Dusek et al., 2006; Köhler, 1936). The
initial formation of ice crystals in mixed-phase clouds is due to the
ability of an ice-nucleating particle (INP) to induce freezing in a
supercooled liquid droplet via heterogeneous ice nucleation predominantly
through the immersion freezing mechanism (Knopf et al., 2018;
Vali, 2014). Furthermore, ice crystals grow at the expense of water droplets
in mixed-phase clouds because the saturation water vapour pressure is lower
over ice than over water (Korolev, 2007). In this
study, we chose to combine cloud condensation and ice nucleation (IN)
experiments on the same particle type to elucidate effects on mixed-phase
clouds with implications for the subtle balance existing between supercooled
water droplets and ice crystals.
This study focuses on the impact of atmospheric photochemical processing
during the 1-week lifetime equivalent of an organic aerosol particle.
Indeed, in the time interval between where organic aerosols are emitted
and/or formed and where they act as CCN and INPs, they will undoubtedly be
exposed to sunlight and thus undergo atmospheric processing through
photochemistry (George
et al., 2015; Laskin et al., 2015). This atmospheric ageing process modifies
the physical and chemical properties of organic aerosols and subsequently
affects their cloud-forming ability. In fact, atmospheric photochemical
processing of organic aerosols has been shown to increase CCN ability (Slade
et al., 2017; Wong et al., 2011). However, the effect of photochemical
processing of organic matter on ice nucleation is unknown, although recent
work on pollen in deposition freezing mode suggests a decrease in ice
activity (Gute and Abbatt, 2018). Our study links a
photochemical mechanism with CCN and with IN efficiencies applicable for an
atmospheric organic aerosol.
Aquatic dissolved organic matter (DOM) is known to be highly photoreactive
by direct and indirect photochemical processes (McNeill
and Canonica, 2016; Rosario-Ortiz and Canonica, 2016; Sharpless et al.,
2014; Wenk et al., 2013). DOM therefore allows us to probe the impact of
photochemistry on the cloud-forming properties of organic matter, while
taking advantage of its known photochemical processes in aquatic
environments. Furthermore, both DOM and organic aerosols possess absorbing
molecules in the visible spectrum, often termed brown carbon
(Laskin et al., 2015). Both materials are also naturally
occurring and contain complex organic molecules, with DOM having more
chemical function group diversity (Graber
and Rudich, 2006; Kristensen et al., 2015).
Materials and methodsSample description and storage
The DOM samples were collected from the Great Dismal Swamp, in Virginia,
USA, and from the Suwannee River, in Florida, USA. The Great Dismal Swamp
was sampled in Suffolk (36.7∘ S, 76.4∘ W) in 2014
(Sun et al., 2014) and from Jericho Ditch
(36.7∘ S, 76.4∘ W) in 2016
(Lin et al., 2017). Suwannee River water
was collected in 2017 close to the collection site of the International
Humic Standard Society (IHSS) (30.5∘ N, 82.5∘ W). The
Great Dismal Swamp and the Suwannee River samples were filtered through a
pre-cleaned 0.2 µm capsule filter (Polycap TC, Whatman) on-site
following collection and kept refrigerated until use. The samples were never
frozen to avoid potential freeze–thaw cycle artefacts. Experimental
solutions with concentrations of approximately 20 mg C L-1 were
prepared upon dilution of the field-collected waters in nanopure water.
The Suwannee River Fulvic Acid Standard II (2S101F) was purchased from the IHSS
and dissolved in nanopure water with concentrations of approximately 20 mg C L-1. The inorganic ions present in Great Dismal Swamp water have been previously reported (Johannesson et al., 2004), and anionic ion chromatography confirmed similar concentrations.
Indeed, DS2014 had concentrations of F-, Cl-, SO42- and
NO3- for a total of 30 µM, which we can approximate to 60 µM of ions assuming charge balance. These ion concentrations suggest
that the ratio of organic to inorganic ions is typically less than 10 % by
molar concentration.
In addition, the DOM sampled for this study was bulk water-collected and
therefore represents an alternative sampling procedure to access organic
carbon compared to impacting particular matter on filters. Indeed, DOM was
not extracted from a filter or from an impinger but rather directly
collected in the field. Ultimately, the photochemical mechanisms identified
in this study are expected to be similar between DOM and organic aerosols,
thereby bridging the fields of aquatic and atmospheric chemistry of organic
matter.
Photochemical setup
Photolysis experiments were conducted in a commercial photoreactor (Rayonet,
Southern New England Ultraviolet Co.) equipped with a motorized turntable
and 6×300 nm light bulbs (UVB – 3000 Å from Southern New
England Ultraviolet Co.). These UVB bulbs have a relative light intensity as
a function of wavelength shown in Fig. S2. A total of 9 mL of a 20 mg C L-1 DOM or
IHSS isolate solution was pipetted into cork-capped 10 mL borosilicate test
tubes (Pyrex, 15 mm × 85 mm, disposable) and irradiated for up to 25 h. During this period, the temperature inside the photoreactor was held at
30–32 ∘C, and the reactor was turned on prior to the beginning of
the experiment to ensure a constant temperate exposure. At each time point,
a test tube was removed from the photoreactor, and the solution was directly
used for either CCN (9–18 mL), INP (9–18 mL), total organic carbon (TOC; 7 mL), ion chromatography
(1 mL) or conductivity (0.1 mL) measurements.
A slightly different experimental setup was used for the CO and CO2
measurements. In order to avoid CO2 partitioning to the gas phase, the
borosilicate test tubes were prepared headspace-free using rubber septa. At
each time point, a test tube was withdrawn from the photoreactor and 6 mL of
the experimental solution was transferred to a N2-flushed 20 mL serum
vial containing 100 µL HCl 1 N. The serum vial was briefly shaken and
stored no longer than a week in the fridge until headspace analysis on the
gas
chromatograph equipped with a flame ionization detector (GC-FID).
Photochemical control experiments
Control experiments were conducted to unambiguously attribute the change in
CCN and INPs to photochemistry. Importantly, the photomineralization
mechanism could have led to the accumulation of carbonate ions in solution
and thus increase the CCN ability of the material with UVB exposure time. As
a control, inorganic carbon measurements by the Shimadzu TOC analyser showed
no increasing IC fraction with irradiation. In addition, control experiments
where the DOM solutions were purged with argon showed no difference in CCN
activity, indicating no contribution to accumulating dissolved carbonate in
solution.
To address the issue of the potential impact of a 30 ∘C heat
exposure on CCN and INPs during the photochemical experiment, foil-covered
test tubes were placed inside the photoreactor alongside non-covered test
tubes. Indeed, the non-irradiated sample that would have experienced the
same temperature elevation from room temperature to 30 ∘C did
not show any change in hygroscopicity, INP activity, organic carbon or
production of photochemical adducts.
Control experiments involving Ar-purged solutions further emphasize that
dissolved O2 is necessary for photomineralization, consistent with
previous reports (Schmitt-Kopplin et al.,
1998). In an atmospheric context, O2 will be readily available to the
aqueous aerosol phase, and we expect in situ aqueous photomineralization of
organic aerosols. Furthermore, as confirmed by dark control experiments,
exposure to light was necessary to drive the chemical, cloud droplet
formation and ice nucleation changes observed.
Actinometry experiments for relating UVB exposure to sunlight
The light intensity inside the photoreactor was monitored with the chemical
actinometer pyridine/p-nitroanisole (PNA)
(Laszakovits et al., 2017).
Briefly, a solution containing 20 µM of recrystallized PNA and 0.25 mM of pyridine in nanopure water was irradiated for 5 h in the experimental
conditions described above. Then, the PNA and pyridine were quantified via
ultra-high-pressure liquid chromatography (Waters Acquity) equipped with a
C18 column (BEH130 C18, 1.7 µm; 2.1 mm × 150 mm) and a
photodiode array detector. The analyses were performed using a 40 : 60 A : B
eluent mixture of high-performance liquid chromatography (HPLC)-grade solvent (A = 90 % (acetate buffer pH 6) + 10 % acetonitrile; B = 100 % acetonitrile), 5 µL injection
volume and 0.2 mL min-1 flow rate. At these conditions, PNA was eluted at 4.2 min (detection at 310 nm) and pyridine at 2.7 min (detection at 250 nm).
The absolute spectral irradiance (Iλ) was obtained according to
Iλ=s⋅Iλ,m, where Iλ,m (J nm-1 m-2 s-1) is the spectral irradiance
measured in the photoreactor with a calibrated Jaz spectrophotometer (Ocean
Optics) using 2 × 300 nm bulbs (Fig. S2). Iλ,m was also
corrected for the absorption of the borosilicate glass occurring between
280 and 300 nm. The scaling factor s was calculated according to Eq. (1):
s=kdeg,PNAPNA0lΦdeg,PNA∑λIλ,mfλ,PNAΔλ,
where kdeg,PNA=(0.37±0.02) h-1 is the
pseudo-first-order PNA degradation rate constant measured with 6×300 nm bulbs, [PNA]0=(17.4±0.4)µM is the starting
PNA concentration, ΦPNA=0.076±0.008 is the direct
photolysis quantum yield calculated according to
Laszakovits et al. (2017),
fλ,PNA is the absorptivity of the starting PNA solution, l is
the path length (l=1 cm) and Δλ=1 nm.
kdeg,PNA, [PNA]0 and ΦPNA are the average values
measured for different days. fλ,PNA represents the fraction
of light absorbed by the PNA starting solution, and it was calculated
according to fλ,PNA=1-10-ελ,PNAPNA0l=1-10-Aλ,PNA, where ελ,PNA is the molar
extinction coefficient of PNA and Aλ,PNA is the absorbance.
This calculation provided an integrated irradiance (280–400 nm) of 64±4 J m-2 s-1 for 6×300 nm bulbs. The
photoreactor-to-sunlight conversion factor, that is the irradiation time in
the photoreactor equivalent to 1 h in natural sunlight, was obtained as
the ratio of the rates of light absorption of a given DOM for the two
different light sources (Rphotoreactorabs, Rsolarabs),
according to Eq. (2).
conversionfactor=RphotoreactorabsRsolarabs
In order to get Rsolarabs, we simulated a solar spectrum (global horizontal irradiance, 300–400 nm, integrated irradiance of 59.2 J s-1 m-2) using SMARTS 2.9.5 (NREL) for a generic point at mid-latitudes (45∘) in summer (1 June, from 05:00 a.m. to 07:00 p.m.).
Using the daily average value of 2.2 during daytime, 25 h of irradiation
in the photoreactor with six UVB bulbs corresponds to 55 h of sunlight
irradiation, which is equivalent to 4.6 d in the environment (assuming
12 daily hours of light on a clear day). The actinometry experiments are
independent of temperature, as they depend on photon flux.
Analytical chemistry instrumentsTOC analyser
The total organic carbon, specifically the non-purgeable organic carbon
(NPOC), and the total inorganic carbon (IC, carbonates) were quantified
using a total organic carbon (TOC) analyser (Shimadzu, model TOC-L CSH). The
NPOC method used 50 µL injections for a minimum of triplicate
measurements and reports concentrations in milligrams of carbon per litre (mg C L-1) with standard
deviations from the average. The method purge time and gas flow were 1.5 min
and 80 mL, respectively. NPOC calibrations were done with recrystallized
potassium phthalate. The IC calibration was done with potassium carbonate
(K2CO3) solutions, and the IC method also used 50 µL
injections for a minimum of triplicate measurements. To confidently
attribute the decreasing TOC value observed during irradiation to the
formation of gaseous products (CO and CO2), we ran solutions containing
formaldehyde, formic acid, acetone and urea with the NPOC method, and we
confirmed that those compounds were not removed from the solutions during
the purging time within the TOC instrument.
Conductivity measurements
Conductivity measurements were made using a portable Horiba Scientific
LAQUAtwin model B-771 conductivity kit. The conductivity probe was
calibrated with the manufacturer's standard solution at 1.41 mS cm-1.
GC-FID measurements
CO and CO2 headspace concentrations were quantified by a gas
chromatograph (GC) equipped with a flame ionization detector (FID) and a
methanizer (model 8610, SRI instruments, Menlo Park, CA). The methanizer
ensures high sensitivity following separation over a 2.7 m HayeSep D column
with N2 carrier gas. Column and detector temperatures were 40
and 300 ∘C, respectively. Under these conditions,
CO and CO2 eluted at 1.82 and 4.27 min, respectively. From the
headspace concentration (pA,hs, where A is CO or CO2),
the aqueous-phase concentration in the photolysis test tube
([A]aq0) was calculated with Eq. (3).
Aaq0=nA,aq+nA,hsVaq,
where Vaq is the volume of liquid in the 20 mL serum vial, and
nA,aq and nA,hs are the moles of CO2 in
the aqueous phase and in the headspace, respectively. nA,aq
was calculated from the measured headspace vapour pressure
(pA,hs) via Henry's law, while nA,hs was also
obtained from pA,hs using the ideal gas law and Eq. (4).
nA,aq=pA,hsKA⋅VaqKA is Henry's constant corrected for the temperature T at
the time of the measurement (Fry et al., 1995), R is the gas constant and Vhs,corr is the headspace
volume corrected for the vial overpressure (Vhs,corr=Vhs⋅ptot,hspatm, where ptot,hs is the total gas pressure in
the vial before the GC measurement).
Low-molecular-weight organic acid analysis
Acetic acid, formic acid, oxalic acid and pyruvic acid were quantified via
ion chromatography (DX-320, Thermo Scientific, Sunnyvale, CA, USA). The
instrument was equipped with an EG40 eluent gradient generator, a Dionex IonPac AG11-HC RFIC 4 mm column and guard column, a Dionex AERS 500 4 mm
electric suppressor and an electrical conductivity detector. Injection
volume and flow rate were 250 µL and 1.5 mL min-1, respectively.
The following KOH gradient was used: 0–11 min, 1 mmol L-1; 11–37 min, 1 mmol L-1 to 40 mmol L-1; 37 to 38 min, 40 mmol L-1; 38
to 41 min, 1 mmol L-1. In these conditions, acetic acid, formic acid,
pyruvic acid and oxalic acid were eluted at 9.4, 11.9, 13.0 and 24.9 min,
respectively.
Cloud chamber instruments and techniquesCloud condensation nuclei counter setup
The DOM samples were atomized by a home-made atomizer (based on a TSI
aerosol generator) from a heart-shaped glass flask to minimize the amount
of solution required for efficient aerosolization. The generated flow of
polydispersed wet aerosols was then dried through orange silica gel and 4 Å molecular sieves; the exiting flow had a relative humidity <5 %. The polydisperse flow of dry aerosols was used to generate a
monodisperse flow with a TSI differential mobility analyser (DMA; TSI model
3082) operating with a sheath-to-sample flow ratio of 10:1. The
monodispersed dry aerosol flow was subsequently split in two and sampled by
a TSI condensation particle counter (CPC-3772) and a Droplet Measurement Technologies cloud condensation
nuclei chamber (CCNC-100) (Roberts
and Nenes, 2005; Rose et al., 2008). The CCNC operated with a total flow of
0.5 L min-1, corresponding to a high RH exposure time of approximately 10 s for
the aerosols. We do not expect dynamic surface tension changes on this timescale (Nozière et al., 2014). The number of
activated droplets measured by the CCNC is then divided by the total number
of particles measured by the CPC to yield the activated fraction and
supersaturation (SS) curves. Specifically, the CCNC was operated by
generating aerosol particles of a fixed dry diameter (80 nm) while changing
supersaturations (SS) (0.2 % to 1.0 %) as well as using a fixed SS
(0.40 %) while changing diameters (20 to 120 nm) to ensure
reproducible measurements. Both methods were used to quantify the ability of
DOM to act as a CCN by determining the κ values
(Fig. 1). The resulting κ values were used for
the variance depicted in Fig. 1, as triplicates
were not conducted for every single time point because of limited samples.
From Eq. (5) of the supercritical saturation (Sc), the hygroscopicity
parameter κ is obtained following Eq. (6) (Petters
and Kreidenweis, 2007; Ruehl et al., 2016):
5Sc=D3-Dd3D3-Dd31-κexp4σs/aMwRTρwD,6κ=427Dd3(lnSc)24σs/aMwRTρw3,
where Sc is the supercritical saturation, D is the wet diameter (fitted
parameter when using Eq. 6, Dd is the dry diameter (measured in this
work by the DMA), κ is the hygroscopicity parameter (unitless),
σs/a is the surface tension of water (0.072 J m-2),
Mw is the molecular weight of water (18 g mol-1), R is the
universal gas constant (8.314 J K-1 mol-1), T is the temperature
of the inlet flow (298 K) and ρw is the density of water
(106 g m-3) (Petters and
Kreidenweis, 2007).
The mass of the DOM samples is >90 % organic carbon by molar
concentration, and thus phase separation between inorganic ions and the
organic phase is not likely occurring during aerosolization of the DOM. In
other words, there is no thin film being generated, and thus the δorg parameter, or the thickness of the organic film surrounding an
inorganic core, is not calculated.
Two CCN and INP experiments performed 1 year apart showed reproducible
data, indicating little degradation of the cloud droplet and ice-nucleating
activity due to storage.
Ice nucleation setup
The DRoplet Ice Nuclei Counter Zurich (DRINCZ) is a custom-built instrument
that enables the quantification of heterogeneous ice nucleation through
immersion freezing (David et
al., 2019). Briefly, DRINCZ is an immersion freezing technique with an
optical detection employed in a cooling bath (Lauda Proline RP 845,
Lauda-Königshofen, Germany), a visible light camera
(Microsoft Lifecam HD-3000), and a 96-well sterilized and sealed plate (PCR
tray, 732-2386, VWR, USA). Each well of the plate is filled with 50 µL of the DOM solution using a multi-pipette with disposable tips. Above the
cooling bath, a custom-machined aluminium plate holder keeps the plate in
place so that the liquid in the wells is exactly level with the ethanol
coolant. Additionally, a solenoid valve, triggered by a fluid level sensor,
allows for additional ethanol, pre-cooled to 0 ∘C, to be poured
into the bath to compensate for the solvent's changing density with cooling
and to ensure a constant ethanol level submerging the plate. A thick acrylic
plate on top of the well plate weighs it down and prevents buckling of the
plate to ensure that all the wells are immersed equally and experience the
same temperature. The bath temperature is cooled at a rate of 1 ∘C min-1. A diffuse LED array submerged at the bottom of the ethanol
bath illuminates the plate from below and does not interfere with the
coolant flow. The camera is mounted above the plate and captures the light
transmission through the wells every 15 s. The position of each well on the
image is later automatically obtained using image recognition code in
MATLAB, and the change in light intensity for each well is obtained per
image. Freezing fractions are then obtained for each well by taking the
temperature at the time the image was taken and the instant when 60 %, of
greatest change in light intensity occurred.
The DRINCZ measurement technique is limited to samples that freeze at
temperatures warmer than -22.5∘C, since 50 % of the wells of
the background molecular biology reagent water (89079-460, Sigma-Aldrich,
USA) freeze at that temperature (T50=-22.5∘C). This
detection limit of homogeneous freezing is largely due to well plate
characteristics, which do not change for different well volumes; to water
quality (Polen et al., 2018); and to a sterile
working environment. While others have noted degradation of INP activity
with sample storage (Stopelli et al.,
2014), we did not observe a storage effect on the INP properties, likely
because our DOM samples were filtered on-site to 0.20 µm using sterile
filters and stored at 4 ∘C. Following repeated Sigma-Aldrich
water runs on DRINCZ, a master water background curve was generated.
Specifically, 10 curves were used to generated the background in
Fig. 2a and lead to an average standard deviation
in the freezing temperature detection of 0.2 ∘C. The background
was corrected following (Vali, 1971,
2019), where the nucleus concentration, Nuc(T), of the background is
subtracted from that of the sample and then inverted to reconstruct the
corrected frozen fraction (FF) following Eq. (7) (David et al., 2019).
Nuc(T)=-1VwellΔTln1-ΔNN(T),
where Vwell is the aliquot volume of 50 µL, ΔT is the
change in temperature between each recorded image, N(T) is the number of
unfrozen aliquots at the beginning of a temperature step, and ΔN is the
number of aliquots that freeze during the temperature step and, in DRINCZ's
case, between recorded images. All reported FF curves have been background
corrected.
The CCN ability of DOM and of Suwannee River fulvic acid (SRFA) increases as a function of
irradiation. The experimental conditions are equivalent to up to 25 h of UVB
exposure (∼55 h of sunlight in the atmosphere). The
colours indicate different source material (DS 2014 in red, DS 2016 in
purple, SR 2017 in blue and SRFA in black), and the symbols indicate the
material was field-collected (circles) or commercially available (squares).
The instrumental uncertainty in κ=±0.01. To show the
spread of the measurements, the variance from multiple independent
experiments are illustrated as bars. The variance was used since not every
point had triplicates for standard deviations to be calculated.
DOM is IN active. (a) The frozen fractions (FFs) as a function of
temperature are displayed for field-collected DOM from the Great Dismal Swamp,
Virginia, USA, in 2014 (DS 2014, red) and in 2016 (DS 2016, purple); from the
Suwannee River, Florida, USA, in 2017 (SR 2017, blue); and for the reference
material Suwannee River fulvic acid (SR fulvic acid, black). The solid
50 % frozen fraction line is used to determine the T50, the
temperature at which 50 % of the droplets are frozen. Specifically, DS
2014, DS 2016, SR 2017 and SRFA showed T50 values of -12.4±0.1, -7.8±0.3, -9.4±0.1 and -13.0±0.3∘C, respectively. The black
line represents the water background FF curve with standard deviation (1σ) from 10 different water backgrounds run over a period of 6 months.
(b)T50 as a function of UVB exposure for all DOM samples is shown. All
samples exhibit a decrease in IN activity upon 25 h of UVB exposure
(∼55 h of sunlight in the atmosphere). The lines are
linear fits to the data to quantify the slope and the rate loss of IN
activity. The slopes are -0.054, -0.145, -0.066 and -0.100 for DS 2014,
DS 2016, SR 2017 and SRFA, respectively. (c) The shaded regions in the
nm space correspond to the extent of change in IN activity due to
photochemistry from the mean values at t=0 h to t=25 h. Note that
the nm values were determined by accounting for the change in organic
carbon content measured by TOC analysed and depicted in
Fig. 4. The Wilson 2015 parameterization for
nm of INPs in the sea surface microlayer is also shown for comparison
(Wilson et al., 2015).
The INP per milligram of carbon is the nm value and was calculated following the
quantitative analysis Eq. (8) according to Vali (1971).
nm=-ln1-FFTOC×Vwell,
where FF is the frozen fraction (value between 0 and 1), TOC is the
concentration of non-purgeable organic carbon (mg C L-1) and Vwell
is the aliquot volume (50 µL). The data analysis was coded in MATLAB and in Igor Pro.
Results and discussionPhotochemical ageing of DOM
To study the effect of photochemistry on CCN and INPs, we sampled naturally
occurring DOM from surface waters from the Great Dismal Swamp in Virginia, USA, in
2014 and in 2016 and at the Suwannee River, Florida, USA, in 2017. The
natural DOM sampled from freshwater bodies is representative of a fraction
of atmospheric organic aerosols directly relevant to lake spray aerosols and
potentially prevalent in the aerosol phase over large freshwater bodies
such as the Laurentian Great Lakes
(Axson
et al., 2016; Slade et al., 2010), the African Great Lakes and Lake Baikal.
Furthermore, we used commercially available Suwannee River fulvic acid
(SRFA), an established standard for complex organic matter and humic-like
substances in both aquatic
(Cory
et al., 2009; Guerard et al., 2009) and atmospheric chemistry experiments
(Chen and Valsaraj, 2007; Dinar et al., 2006; Frosch et al., 2011; Kessler et al.,
2012; Ladino et al., 2016; Slade et al., 2017; Svenningsson et al., 2006;
Wang and Knopf, 2011; Zelenay et al., 2011). To further bridge the gaps of
aquatic and atmospheric chemistry, we used the same concentrations typically
found in rivers and in cloud water, that is 20 mg L-1
(Cook
et al., 2017; Johannesson et al., 2004). Organic aerosols containing
humic-like substances may have lower average molecular weights and a lower
aromatic moiety content than SRFA standards
(Graber and Rudich, 2006), yet we
complement our study with real surface-collected waters to circumvent IHSS
extraction and processing of DOM. Ultimately, we argue that our DOM samples
are naturally occurring in freshwater bodies and are adequate and alternative
proxies for visualizing the photomineralization mechanism identified in this
study.
Bulk DOM samples were irradiated for 25 h with UVB light in a photoreactor
in order to simulate the photooxidation processes occurring in the
atmosphere. Using a combination of chemical actinometry and radiometry
measurements, we estimate that our 25 h exposure experiments in the
photoreactor are equivalent to approximately 55 h of sunlight
irradiation, which is further equivalent to 4.6 d in the environment. However, all
experiments were conducted at 30–32 ∘C, and an extrapolation
to colder and more relevant atmospheric temperatures would slow the rates of
reactions of photochemical processes but would not affect the photon flux.
Many photochemical processes have weak temperature dependences, and for
instance hydrogen peroxide quantum yields are affected by a factor of 1.8
per 10 ∘C (Kieber et al., 2014). Thus, the
exposure time corrected for lower temperature is relevant in the context of
an approximately 1-week aerosol lifetime in the atmosphere.
The bulk solutions of irradiated DOM were subsequently analysed for CCN and
INP activity. In all of our experiments, we employed concentrations between
14 and 20 mg of carbon per litre (mg C L-1), equivalent to 1160 to 1670 µM of organic carbon, which represents typical aquatic DOM conditions
(Johannesson et
al., 2004) as well as typical concentrations found in cloud water
(Cook et al., 2017). An important
caveat to highlight here is that our photochemical experiments were
conducted in bulk. It is thus likely that the effect quantified does not
necessarily equate to identical processes in the droplet and/or in the
organic aerosol phase. However, we expect the photoproducts and mechanisms
within the bulk to be occurring within an aerosol droplet, potentially at
faster rates because of higher organic concentrations in the aqueous aerosol
versus in the bulk. In addition, organic aerosols can also exist under
different mixing states, phases and morphologies likely leading to matrix
effects which were not captured in this work
(Lignell et al., 2014; McDow et
al., 1996).
Photochemistry affects the hygroscopicity of DOM
To assess the ability of DOM to act as CCN, the κ values of each
irradiated DOM sample were calculated, where higher κ values indicate
a more effective CCN. During irradiation, we observed a clear increase in
κ values, up to 2.5 times higher than the non-irradiated samples
(Fig. 1). The change in CCN activity with increased
UVB exposure was systematically observed across all of the naturally
occurring samples tested, including the SRFA standard. In this study,
κ values for photochemically exposed DOM ranged between 0.12 and
0.45 with an instrumental error of 0.01 and an experimental uncertainty of
less than 15 %. These values compare to global ambient organic aerosol
which has κ values between 0.15 and 0.30
(Schmale et al., 2017). We further note the seemingly
large κ values for organic matter, specifically for the irradiated
field-collected samples DS 2014, DS 2016 and SR 2017
(Fig. 1). We suggest that as the organic matter is
photomineralized, the organic-to-inorganic ratio decreases, increasing
κ values. Organic acids are also being formed and can contribute to
increased κ values (see further discussion in Sect. 3.4.2).
According to
Mikhailov et al. (2013), κ is inversely proportional to molar mass, and, at equal particle density and at unity van't Hoff factor, a decrease in molar
mass would lead to an increase in κ.
In particular, SRFA's CCN activity has been reported to have κ
values of 0.06–0.08
(Dinar et al.,
2006; Slade et al., 2017). In our experimental setup, SRFA has a κ
of 0.12±0.02, and we attribute part of this discrepancy to the lower
concentrations and the complete solubility of 20 mg C L-1 of SRFA used
in our experiments. Concentrations above 5000 mg C L-1 were previously
used, and such concentrations form a suspension rather than a homogeneously
dissolved solution potentially leading to lower κ
(Dinar et al.,
2006; Slade et al., 2017). This possible concentration effect on CCN
activity could arise from the interfacial molecular arrangement of the
organic polymeric material (Ruehl et
al., 2016), which leads to changes in surface tension and thus to κ
values (Nozière et al., 2014;
Ovadnevaite et al., 2017).
Photochemistry affects the ice-nucleating ability of DOM
To probe the impact of photochemistry on ice nucleation, the field-collected
DOM and SRFA were measured in our home-built DRoplet Ice Nuclei Counter
Zurich (DRINCZ) (David et al.,
2019). We report median freezing temperature (T50) values which
correspond to the temperature at which 50 % of the wells filled with a
DOM solution froze (Fig. 2a). The natural DOM
samples from different locations and different years show frozen fractions (FFs) significantly above the background ultra-pure water, demonstrating ice
nucleation activity (Fig. 2a). Specifically, DS
2014, DS 2016, SR 2017 and SRFA showed T50 values of -12.4±0.1, -7.8±0.3, -9.4±0.1 and -13.0±0.3∘C, respectively, which is
well above the T50 value of ultra-pure water in DRINCZ (-22.5∘C). The steep curve of the FF curves exhibited by the samples suggests that the nature of the ice-nucleating material or active site is
relatively homogeneous. These freezing temperatures are consistent with
recently identified river nanoscale INPs
(Knackstedt
et al., 2018; Moffett et al., 2018). The FFs were then normalized by the
organic carbon concentration in the solution measured by a TOC analyser to
yield nm values as a function of temperature (Fig. S1). These
nm values physically describe the ice nucleation active mass in units of
number per milligram of carbon. We compare our nm values with those of the
Wilson 2015 parameterization which represents the ice-nucleating ability in
the immersion freezing mode of primarily organic material in the sea surface
microlayer (Wilson et al., 2015) (Figs. 2c and S1).
This comparison suggests that our DOM samples, which are smaller than 0.20 µm, could be accounting for a subset of the IN activity previously
observed in the sea surface microlayer
(Irish et al., 2019; McCluskey et al., 2018; Wilson et al., 2015). Furthermore, sea
spray aerosols have recently been shown to be effective INPs when enriched
in organics (DeMott
et al., 2016; Ladino et al., 2016; Si et al., 2018; Wilson et al., 2015),
consistent with our results. Soil organic matter, also containing humic-like
substances, is also able to act as INPs in the immersion freezing mode
(Hill
et al., 2016; O'Sullivan et al., 2014). The SRFA was observed to have lower
IN activity than the collected DOM samples, despite SR 2017 having been
collected near the IHSS collection point at Suwannee River. This result
suggests that extraction and processing of Suwannee River DOM to yield its
fulvic acid component reduces its IN activity, potentially related to
differences in organic carbon content
(Botero et al., 2018). In
other words, the fulvic acid is not as good as an INP compared to other components
within DOM. Our results for SRFA are consistent with heterogeneous ice
nucleation studies which used aerosolized SRFA in deposition freezing mode
and found the organic material to be active
(Wang and Knopf, 2011). Ultimately, DOM
has the potential to nucleate ice at low to moderate (-4 to -12∘C) droplet supercooling through immersion freezing.
The DOM samples were then subjected to photochemical exposure in an
identical setup to the CCN experiments, and the irradiated samples were
measured on DRINCZ. Photochemistry decreased T50 values as a function of
UVB irradiation, with an important impact on corresponding nm values
(Fig. 2). Indeed, the 2 to 3 ∘C change
in the T50 temperature observed in Fig. 2b
leads to corresponding changes of up to 2 orders of magnitude in nm
values between the 0 and the 25 h time points in
Fig. 2c. In particular, for an nm of 100 ice
nucleation sites per milligram of carbon, DS 2016 experienced a suppression in IN
activity of up to 4 ∘C (Fig. 2c). The
irradiated DOM samples led to different spreads of nm values, indicating
DOM's composition-specific photochemistry response on immersion freezing, and
warrants further study. Furthermore, the rate loss of IN activity measured
as T50 could be quantified as a function of hours of UVB exposure. The
slopes for photochemically exposed DS 2014, DS 2016, SR 2017 and SRFA were
-0.054, -0.145, -0.066 and -0.100∘CT50 h-1,
respectively (Fig. 2b). The average decrease in
T50 values for irradiated DOM samples is thus -0.09∘CT50 h-1 of UVB light and equates to -0.04∘CT50 h-1 of sunlight at ground level in mid-latitudes (see methods for
conversion of photochemical reactor exposure to sunlight exposure). These
values could be useful to the atmospheric modelling community to investigate
a lifetime-dependent IN activity of organic matter such as DOM or sea spray
aerosols.
Although our study is the first to quantify the effect of photomineralization on the ice-nucleating ability of organic matter, others have identified
specific organic molecules and components that could help explain the IN
ability of DOM. Wilson et al. parameterized the ice-nucleating ability of
sea surface microlayer samples using TOC and temperature
(Wilson et al., 2015). However, this
parameterization leads to higher estimates of nm than obtained for our
DOM samples (Fig. 2c). This result suggests that
nm does not efficiently constrain the IN activity of DOM. McCluskey et
al. (2018) arrived at a similar conclusion for the IN activity of sea spray aerosol.
The INPs measured in this study were soluble with an operational definition
of passing through a 0.2 µm filter and could account for a subset of
INPs identified in marine samples. In addition, soluble INPs have been shown
to contain extracts from plant-based material, including cellulose
(Hiranuma et al., 2015) and
proteinaceous material
(Augustin
et al., 2013; Dreischmeier et al., 2017; Koop and Zobrist, 2009; O'Sullivan
et al., 2015; Pummer et al., 2012, 2015; Wilson et al., 2015), which we know
are present within our DOM samples. Finally, Gute and Abbatt observed that
OH radical oxidation of pollen in deposition freezing required higher
supersaturation ratio with respect to ice (Gute and
Abbatt, 2018). Our results nicely corroborate with this observation and
further suggest that not only OH radicals but photochemical processes in
general are able to decrease the ice nucleation ability of organics,
including pollen, in more than one freezing mode.
Chemical changes occur due to photochemistry
To understand the molecular origin of the observed photochemical impacts of
DOM on cloud droplet (Fig. 1) and ice nucleation
(Fig. 2), we tracked total carbon, absorbance,
conductivity, pH, organic acids, CO and CO2 during irradiation.
Photooxidation products are formed from DOM. This graph depicts
data from (a) DS 2014 and SRFA and from (b) DS 2016 and SR 2017. CO2
and CO were quantified via GC-FID, whereas formic, acetic, oxalic and
pyruvic acids were quantified by ion chromatography. Pyruvic acid for SRFA
was below detection limits. Note that the concentrations are expressed per
moles of carbon, and so organic acids with multiple carbon atoms have been
normalized.
CO and CO2
The photooxidation products CO and CO2 were quantified by gas
chromatography with a flame ionization detector (GC-FID) equipped with a
methanizer between the GC column and the FID detector. The major and final
product was CO2, accounting for 50 % to 100 % of dissolved organic
carbon mass loss (Fig. 3). On the other hand, CO
accounted for no more than 0.1 % mass yield, indicating that the original
organic carbon content of DOM is removed primarily by photomineralization to
CO2 after 25 h of UVB irradiation (Fig. 3). It
must be noted that the CO and CO2 concentrations reported here
represent a lower limit, as these particular irradiation experiments needed
to be performed in a headspace-free setting to limit analyte loss. This
setup consequently limits dissolved O2 availability, potentially
limiting the extent of oxidation. However, this limitation is not expected
to occur in liquid atmospheric aerosols. The CO2 yields presented in
this study serve as experimental evidence for the ultimate fate of the
organic carbon aerosol pool: the production of CO2.
Organic acids and pH
Formic, acetic, oxalic and pyruvic acid are known DOM photooxidation
products (Moran and Zepp, 1997)
and have been previously quantified in organic aerosols, further supporting
their identification as tracers for photooxidation
(Boreddy et al., 2017; Pillar and Guzman, 2017; Zhang et al., 2016). All four organic
acids were produced during the photooxidation of all the DOM samples
(Fig. 3). In addition, oxalate has been recently
quantified to be a major photoproduct of dissolved organic carbon from
primary wildfire emissions
(Tomaz et al., 2018).
Formic acid and acetic acid concentrations increase continuously, but oxalic
acid and pyruvic acid concentrations show a growth-and-decay profile
(Fig. 3). Indeed, pyruvic acid is a known
intermediate photoproduct on the pathway to the mineralization products of
CO2 (Griffith et al.,
2013).
The quantified production of formic, acetic, oxalic and pyruvic acids
supports the changing chemistry observed during aqueous photooxidation
related to the changes in CCN; a high O/C ratio leads to higher κ
values. It is clear that the same oxidation products are produced, and thus
the same photomineralization mechanism is operating in all DOM samples,
albeit to different extents. This difference is attributed to the ability of
the DOM samples to absorb irradiation and to generate different amounts of
reactive oxygen species. These acids also do not appear to increase the
efficiency of DOM to act as an INP, suggesting a significant role of the
larger molecules within DOM to nucleate ice by immersion freezing.
In addition, the pH of the solutions was measured to be approximately five
at each time point for the field-collected DOM samples and remained
unchanged throughout the irradiation period. The pH values further support
the absence of carbonate anions in solution. We further hypothesize that DOM
has a high enough buffering capacity to prevent weak acids from decreasing
the pH during photomineralization.
Total carbon and conductivity
All DOM solutions reproducibly lost organic carbon during UVB exposure
(Fig. 4). The organic carbon loss was dependent on
the DOM sample and ranged between 15 % (SRFA) and 63 % (DS 2014) over
the course of 25 h of UVB exposure, representing 230 to 740 µM of
carbon loss (Fig. 4). Specifically, the κ
value of SRFA upon 25 h of UVB exposure increased the least compared to the
natural DOM sample and can be explained by changes in TOC. Furthermore, the
inorganic carbon fraction was quantified and found to be consistently below
0.3 mg C L-1, indicating no accumulation of carbonate in solution. The
conductivity of the DOM samples remained constant over UVB exposure at 60±14, 19±1, 28±1 and 30 µS cm-1 for DS 2014, DS 2016, SR 2017 and
SRFA, respectively, indicating no meaningful change in ionic species
concentration. The conductivity measurements rule out the hypothesis that
the increase in CCN and the decrease in INP during irradiation is due to the
production of ionic species.
Decrease in the dissolved organic carbon (in both mg C L-1 and in µM) present in DOM upon UVB irradiation. Both the organic (as
non-purgeable organic carbon) and inorganic carbon (as carbonates) were
quantified by a TOC analyser, which was calibrated using standard solutions
of terephthalic acid and carbonate, respectively. Note that dissolved
CO2 is not quantified by TOC analysis. The inorganic carbon is averaged
for all samples with the standard deviation in grey. The percentage values
indicate the conversions of original organic matter into photoproducts.
In our DOM samples, there are insufficient inorganic ions for the organic
matter to act as an organic film when aerosolized. However, photomineralization
leads to a decrease in the organic carbon and a decrease in the
organic-to-inorganic ratio (Fig. 5). The decreasing
organic carbon concentrations at the surface of the droplet may be
increasing the surface tension of the particle, bringing it closer to the
surface tension of water (Kristensen
et al., 2014; Petters and Petters, 2016). Remarkably, decreases in organic
carbon affect all DOM in the exact same way, which is evidenced by the identical
slopes of -0.02 (per mg C L-1) of all κ vs. organic carbon in
Fig. 5. This result supports the notion that the
change in CCN activity is dependent on the decrease in organic carbon
content. Furthermore, the smallest change in κ exhibited by SRFA is
correlated to the smallest change in its organic carbon content in
comparison to the natural DOM samples, indicating a non-negligible impact of
processing on the commercial sample.
CCN abilities and IN abilities of DOM as a function of organic
carbon. Slopes for κ vs. organic carbon are all y=-0.02x.
However, the slopes for T50 vs. organic carbon are different for
different DOM: 0.17, 0.66, 0.34 and 0.81 for DS 2014, DS 2016, SR 2017 and
SRFA, respectively.
Finally, the change in CCN and INPs was correlated with organic carbon
content (Fig. 5). The decrease in INP ability due
to photochemistry (Fig. 5) suggests that the
organic carbon is likely the source of ice nucleation activity. In contrast
to the changes in CCN ability, which all demonstrated identical dependence
on TOC (see Fig. 5), the INP ability dependence on
TOC was variable. Interestingly, Great Dismal Swamp DOM 2014 experienced the
highest organic carbon conversion (Fig. 4), yet its
decreasing T50 values led to a negligible impact on its nm values
(Fig. 2c). Although chromophoric species appear
to be key INP contributors, we conclude that the chromophoric species alone
in this specific DOM sample were not solely responsible for the IN activity.
Specific structures or chemical moieties of the organic content responsible
for the ice nucleation activity still elude us
(Knopf et al., 2018). On the other hand, the
irradiated Great Dismal Swamp DOM 2016 sample experienced the most pronounced
change in T50 values and an intermediate change in organic carbon
content which also led to a large decrease in nm values.
Figure 5 highlights the different linear slopes
observed between T50 values and organic carbon and suggests that total
organic carbon alone is not an accurate parameter for predicting INPs.
Rather, the organic carbon is photomineralized but the ice-active component
of the DOM is being affected to different extents within the different
samples. This result suggests that specific chemical moieties could be
responsible for the immersion freezing activity.
DOM absorbance decreases as a function of irradiation. Absorbance
of a 14 mg C L-1 of Great Dismal Swamp 2014 (DS 2014) as a function of UVB
exposure leads to photobleaching of the DOM.
Overview of the impacts of the photomineralization mechanism on
photochemical changes in DOM and on DOM–cloud interactions. After field-collected DOM and SRFA were exposed to the equivalent of 4.5 d of
sunlight, the TOC concentrations decreased between 15 % and 63 % conversion,
while the production of CO, CO2 and organic acids, including formic
acid, acetic acid, oxalic acid and pyruvic acid, increased, with yields
reported in molar carbon. In addition, the absorbance of the material
decreased with photooxidation leading to photobleaching. These
physicochemical changes are linked to a decrease in IN ability and an
increase in CCN ability.
Absorbance and photobleaching
The DOM material is coloured due to the presence of chromophoric components
within the natural samples. Upon photooxidation, the sample absorbance
decreases as a function of UVB exposure, and photobleaching is observed at
all wavelengths (Fig. 6). This photobleaching
process has also been experimentally characterized for aerosol probe
molecules (Zhao et
al., 2015), for biomass burning aerosols (Wong
et al., 2017) and for α-pinene secondary organic aerosols
(Walhout et al., 2019). We show here that
photobleaching is concurrent with a gain in CCN, a loss in IN activity and a
loss in organic carbon. These results indicate that the chromophoric species
within all the DOM samples are being photomineralized with UVB exposure, and
this mechanism is expected to take place within the organic aerosol 1-week
lifetime in the atmosphere. Our experimental results are further
corroborated by a recent modelling study which proposes an ageing scheme for
photobleaching of organic aerosols to further increase the correlation
between modelled and observed brown carbon absorption
(Wang et al., 2018).
Photochemistry and the photomineralization mechanism
Upon irradiation of DOM, reactive intermediates such as triplet state DOM,
OH radicals, singlet oxygen 1O2 and peroxides can be formed (Manfrin
et al., 2019; McNeill and Canonica, 2016; Rosario-Ortiz and Canonica, 2016).
The reactive oxygen species are produced as a result of indirect
photochemical processes involving the chromophoric components of DOM, which
act as photosensitizers within organic aerosols (Arangio
et al., 2016; Corral Arroyo et al., 2018; Kaur and Anastasio, 2018; Laskin
et al., 2015). The combination of both direct photodegradation and oxidation
by reactive intermediates leads to the photomineralization of the organic
carbon present in DOM (Fig. 7). This process is
ubiquitous in natural aquatic systems, and it represents a competitive form
of dissolved organic carbon processing in surface environments such as
arctic lakes and rivers along with microbial processing
(Miller and Zepp, 1995). It is therefore reasonable to assume
that this process is operating in the photochemical ageing of aqueous organic
aerosols. Nevertheless, a detailed, arrow-pushing chemical mechanism from
complex organic matter to organic acids to CO2 remains elusive.
These photochemical processes are occurring in situ and are not due to
external heterogeneous oxidation. In fact, SRFA heterogeneous oxidation by
OH radicals in a flow tube experiment does not lead to substantial organic
carbon mass loss (Kessler et al.,
2012), indicating the significance of in situ photochemistry compared to
external gas-phase heterogeneous oxidation. Indeed, we show that
photochemical processing can be a dominant atmospheric ageing process,
impacting CCN and INP efficiencies (Fig. 7).
Atmospheric implications
Models of cloud microphysics often use ground-based CCN and INP measurements
for validation studies. Regional models and global climate models which contain prescribed
CCN concentrations may be underestimating their contribution to clouds if
the input is based on data collected and measured at ground level close to
emission sources. The global average of κ is estimated to be 0.3
based on a recently harmonized CCN worldwide dataset
(Schmale et al., 2017). If the CCN activity of the
organic fraction of the aerosol population were to increase with altitude
due to longer photochemical exposure, one should expect a higher CCN
activity of the aerosol population (Fig. 7). A
change in κ values from 0.15 to 0.35, as observed in our own results
for DOM, for a simulated particle of a 100 nm dry diameter, could change the
supercritical saturation conditions for the 50 % CCN activated fraction from
0.30 SS % to 0.20 SS % (above 100 %). The impact of photochemical
ageing would then depend on the aerosol size distribution, but, with a
typical accumulation mode distribution reported by Paramonov et al. (2015), a decrease in the required critical supersaturation from 0.30 SS % to 0.20 SS % to activate the same population of aerosols could
increase the activated fraction from 0.3 to 0.4. This change would lead to
an earlier onset and a higher concentration of cloud droplets in a lifting
air mass, thus forming clouds at lower altitudes with potential effects on
the brightness and optical depth of the cloud. Changes in particle size
during photochemistry could in turn affect the CCN activation conditions;
however, only small reductions in particle diameter have been previously
reported (up to 4 %) for chemical ageing reactions relevant to the
atmosphere (George et al.,
2007).
Whether it is a dust particle covered in an organic coating, a bioaerosol
or a secondary organic aerosol, many INPs contain organic carbon material (Knopf
et al., 2018; Schmidt et al., 2017) and would thus be prone to
photomineralization once in a droplet. The decrease in INP concentrations
with photochemical processing results in a change in T50 of up to 4 ∘C over a 25 h UVB exposure corresponding to a simulated exposure
time of 55 sunlight hours (Fig. 7). This temperature
change could impact the ratio of ice to water droplets within a
mixed-phase cloud by delaying the onset of glaciation and increasing the
supercooled liquid fraction of the cloud due to the lower T50
temperatures, thus modifying the radiative properties and the lifetime of
the cloud. If the decrease in IN activity due to photochemical ageing is
omitted from models, ice formation may occur at higher temperatures,
leading to an earlier more effective glaciation of mixed-phase clouds than
in reality, which has been reported to occur in models
(Lohmann and Neubauer, 2018). Earlier
glaciation has implications for cloud lifetime and precipitation, leading to
an underestimation of the importance of the cloud-phase feedback in
anthropogenic climate change (Tan and
Storelvmo, 2015). Indeed, it has been shown that accurate representation of
supercooled liquid fraction is crucial to predicting climate sensitivity in
a future warming climate (Tan et
al., 2016).
Data availability
Datasets S1, S2, S3 and S4 corresponding to data presented in Figs. 1, 2a, 2b, and 2c, respectively, are deposited in the ETH Research Collection data repository at 10.3929/ethz-b-000342107 (Borduas-Dedekind, 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-12397-2019-supplement.
Author contributions
NBD, ZAK and KM designed the experiments with contributions from ROD and RO.
NBD, RO, LSB, ROD and VW conducted the experiments and collected data. NBD
analysed the data. The manuscript was written by NBD with contributions from
ZAK, RO, ROD and KM.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We acknowledge Kenneth Mopper, Vivian Lin and Paul Erickson
for the field collection of the dissolved organic matter water in 2014, 2016
and 2017, respectively. We acknowledge technical help from Björn Studer and Martin Schroth with the TOC analyser and the GC-FID,
respectively.
Financial support
This research was supported by ETH Zurich and by the Swiss National Science Foundation (grant no. 200020_159809). Nadine Borduas-Dedekind was supported by an NSERC postdoctoral fellowship.
Review statement
This paper was edited by Alexander Laskin and reviewed by two anonymous referees.
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